Compact sources of high-frequency low-jitter picosecond optical pulses are desired for applications such as high-resolution photonic analog-to-digital conversion, arbitrary RF waveform generation, RF-photonic channelization, RF-photonic radar signal processing, coherent optical signal processing, spectrally efficient photonic transmission systems, and photonic synthesis of low phase noise RF tones. For all these applications, the distinguishing feature is the low-jitter characteristic. By way of example, photonic analog-to-digital (A/D) converters (ADC's) have the potential to achieve high resolution, high bandwidth sampling beyond the capability of current electronic ADC's, but require low jitter pulsed optical sources. For example, a 10-bit 10-GS/s photonic A/D system may require an aperture jitter below 30-fs.
Typically, jitter values are inferred from measured phase noise profiles. Phase noise may be measured relative to a high-quality RF source which simultaneously acts as reference and as the active modelocking source for an optical source, using, e.g., commercially available phase noise test sets such as the E5501 manufactured by Agilent, Inc. These residual phase noise profiles are typically characterized by a white noise plateau starting at offset frequencies close in to the carrier (˜10 Hz) and extending to a characteristic roll-off frequency. For harmonically modelocked lasers, this roll-off frequency is much less than Nyquist frequency and the phase noise profile includes the presence of supermode spurs at offset frequencies corresponding to harmonics of the longitudinal mode spacing, a characteristic of harmonically modelocked lasers independent of the gain media used.